MMSET (Multiple Myelomas Set Domain) gene was identified at the breakpoint of the t(4;14) translocation, present in 15-20% of multiple myeloma (MM). This rearrangement leads to overexpression of the FGFR3 and MMSET, however, in about 30% of cases the t(4;14) translocation activates the expression of the MMSET gene alone. This suggests that MMSET is central to the pathogenesis of this form of MM. Our preliminary data indicate that MMSET has properties of a transcriptional co-factor, including nuclear localization, ability to bind chromatin and DNA and the ability to bind sequence specific transcription factors. Furthermore, we found that MMSET has histone methyltransferase activity, modifying histones H3 and H4. These data lead to our overarching hypothesis that aberrant overexpression of MMSET leads to deregulated gene expression in B cells, contributing to pathogenesis of myeloma. In this proposal we will determine the molecular mechanism by which overexpression of MMSET leads to abnormal transcriptional programming and development of multiple myeloma. We will investigate the domains of MMSET that are required to modify chromatin and regulate gene expression. Using a myeloma cell line engineered to delete the overexpressed MMSET allele, we will perform an "add-back" structure-function study. Furthermore, we will investigate the mechanism by which MMSET can cause aberrant gene expression through global changes in histone modifications. Our specific aims include: 1) To determine the enzymatic activity of MMSET and the domains required for chromatin targeting;2) To study MMSET structure and function in a gain-of-function system;3) To determine the mechanisms by which MMSET regulates global histone modification changes. These assays will provide information on the mechanism of transcriptional regulation by MMSET, as well as provide a list of MMSET target genes that may play a role in pathogenesis. Data obtained from this study may lead to new therapeutic strategies for treatment of multiple myeloma.